1. Field of the Invention
This invention relates to a method of blow molding of resin articles and a blow molding system for carrying out the method.
2. Description of the Prior Art
As disclosed, for instance, in Japanese Patent Publication No. 58(1983)-23212, in the blow molding of a resin article, a parison extruded from an extruder is sandwiched between halves of a blow mold and then gas is blown into the blow mold under pressure so that the parison conforms to the inner surface of the mold. Recently, blow molding technique of high-strength material such as reinforced resin has been developed, and there is an increasing possibility of producing structural members by blow molding. However, in the case of a structural member, the parts of the member which supports load must be large in thickness and the thickness must be controlled with a high accuracy, which makes it difficult to produce structural members by blow molding.
For example, when a seat back frame for a car shown in FIG. 11 is blow-molded, opposite side portions must be thicker than the central part. When molding the article, a parison 51 (FIG. 12) is thickened at a pair of predetermined regions 51a diametrically opposed to each other as compared with the other regions 51b.
Such a nonuniformly thickened parison 51 is conventionally obtained by effecting die-shaving (See, for instance, Japanese Unexamined Patent Publication No. 62(1987)-30020.) or core-shaving for an accumulator head of a parison extruder. FIG. 13 shows an accumulator head 55 of a parison extruder provided with a core shaving. As shown in FIG. 13, a parison 51 is extruded downward through an annular gap 54 between a core 52 and a ring die 53. The core 52 is shaved at portions corresponding to the portions of the parison 51 to be thickened, whereby the gaps 54a and 54b are enlarged at the portions. The portions of the parison 51 extruded through the gaps 54a and 54a are larger in thickness than the other portions. When such a parison 51 is blown, the respective portions are stretched and a blown article is obtained. Also in the blown article, the portions of the article formed by the thicker portions of the parison 51 are thicker than the other portions and have desired strength.
However, this method of extruding the parison 51 is disadvantageous in that, when the parison 51 is extruded through accumulator head 55 having such a shaved core, the speed at which the parison is extruded through the gaps 54a and 54b differs from that at which the parison is extruded through the other portion of the gap 54, and due to this difference in extruding speed, the surface of the extruded parison can be corrugated (51c) as shown in FIG. 14 or the parison cannot be straight in shape if the nonuniformity in the thickness exceeds a predetermined limit. Thus in accordance with the known method, a blown article having a large nonuniformity in the thickness cannot be molded.
Further, since, unlike the injection molding, only the outer side of the parison is pressed against the die surface in the blow molding, it becomes very difficult to precisely control the thickness of the wall as the shape of the molded article becomes complicated.